The present invention relates generally to semiconductor-on-insulator (SOI) devices and methods of forming the same and, more particularly, to SOI devices and methods for forming which avoid or reduce floating body effects using a silicon-germanium/silicon heterojunction between the source/drain and the body of a transistor.
Traditional semiconductor-on-insulator (SOI) integrated circuits typically have a silicon substrate having a buried oxide (BOX) layer disposed thereon. A silicon active layer is disposed on the BOX layer. Within the active layer, active devices, such as transistors, are formed in active regions. The size and placement of the active regions are defined by shallow trench isolation (STI) regions. As a result of this arrangement, the active devices are isolated from the substrate by the BOX layer. In addition, a body region of each SOI transistor does not have body contacts and is therefore xe2x80x9cfloating.xe2x80x9d
SOI chips offer potential advantages over bulk chips for the fabrication of high performance integrated circuits for digital circuitry. Such digital circuitry is typically made from partially-depleted metal oxide semiconductor field effect transistors (MOSFETs). In such circuits, dielectric isolation and reduction of parasitic capacitance improve circuit performance, and virtually eliminate latch-up in CMOS circuits. In addition, circuit layout in SOI can be greatly simplified and the packing density greatly increased since.
However, MOSFETs on SOI materials are known to exhibit parasitic effects due to the presence of the floating body. These effects are known as floating body effects and may result in undesirable performance in SOI devices. It will be appreciated from the foregoing that a need exists for SOI MOSFETs having reduced floating body effects.
According to one aspect of the invention, the invention is a method of fabricating a semiconductor-on-insulator (SOI) device having a substrate having a buried oxide layer disposed thereon and an active layer disposed on the buried oxide layer, the active layer having an active region defined by isolation regions, the active region having a source and a drain with a body disposed therebetween, and a gate disposed above the body. The method includes the steps of doping the active layer to form source and drain extensions; forming sidewall spacers adjacent the gate; removing a portion of the active layer in the area of each of the source and the drain, the step of removing a portion of the active layer leaving the extensions disposed under the sidewall spacers intact; forming a silicon-germanium region in each of the source and the drain such that the silicon-germanium regions form heterojunction portions respectively along the source/body junction and the drain/body junction; forming an upper layer of silicon on each of the silicon-germanium regions; and doping the source and the drain with deep implants.
According to another aspect of the invention, the invention is a semiconductor-on-insulator (SOI) device. The SOI device includes a substrate having a buried oxide layer disposed thereon; and an active layer disposed on the buried oxide layer, the active layer having an active region defined by isolation regions, the active region having a source and a drain with a body disposed therebetween, each of the source and the drain having a selectively grown silicon-germanium region disposed under an upper layer of selectively grown silicon, the silicon-germanium regions forming heterojunction portions respectively along the source/body junction and the drain/body junction.
According to another aspect of the invention, the invention is a semiconductor-on-insulator (SOI) device, The SOI device having a substrate having a buried oxide layer disposed thereon; and an active layer disposed on the buried oxide layer, the active layer having an active region defined by isolation regions, the active region having a source and a drain with a body disposed therebetween, one of the source or the drain having a selectively grown silicon-germanium region disposed under an upper layer of selectively grown silicon, the silicon-germanium region forming a heterojunction portion with the body.